1. Field of the Invention
The present invention relates to drilling tools for drilling bore holes in earth formations. More particularly, the present invention provides drilling tools and methods for manufacturing drilling tools which include the use of a new braze alloy for brazing cutting elements in drilling tool applications, especially cutting elements including ultrahard material, such as polycrystalline diamond.
2. Background Art
Earth boring tools, and in particular drill bits, are commonly used in the oil and gas industry for drilling well bores. FIG. 1 shows one example of a conventional drilling system for drilling through earth formation. The drilling system includes a drilling rig 1 used to turn a drill string 2 which extends downward into a well bore 4. Connected to the end of the drill string 2 is a drill bit 6.
Drill bits used for drilling well bores include roller cone drill bits and fixed cutter drill bits. Roller cone drill bits typically include a bit body with an externally threaded connection at one end, and a plurality of roller cones (typically three) rotatably attached to the other end. Attached to the cones of the bit is a plurality of cutting elements typically arranged in rows about the surface of the cones. The cutting elements may be inserts or milled steel teeth. Inserts are typically press-fitted into cavities (or pockets) formed in the surface of the roller cones. In certain applications, a hard surface layer or cap can be brazed onto the milled tooth cones of the roller cone bits. The inserts may include a layer of ultrahard material disposed thereon. Ultrahard material may be bonded to a substrate of the insert in a high temperature high pressure process known in the art.
Fixed cutter drill bits typically include a bit body having a threaded connection at one end and a plurality of cutting elements mounted at the other. The cutting elements are typically mounted in cavities formed around the bit body. In many drill bit designs, these cavities are arranged on blades formed on the bit body to accommodate the placement of the cutting elements on the drill bit. The cutting elements are brazed in the cavities by a braze alloy. The cutting elements typically are compacts which include an ultrahard material, such as polycrystalline diamond (PCD), bonded to a substrate. Ultrahard material may alternatively or additionally include material such as polycrystalline cubic boron nitride (PCBN) or polycrystalline carbonitride (PCN).
Other tools used during drilling operations may also include cutting elements which include ultrahard material, such as polycrystalline diamond or diamond containing material. These tools include other drill bits, reaming tools, fishing tools, milling tools, to name a few.
In general, cutting elements having ultrahard material, in particular diamond containing material, thereon must be brazed at temperatures below 1400° F., and more preferably below 1350° F., to avoid the onset of damage to the ultrahard material. This temperature restriction greatly limits the number of alloys that can be used as braze alloy for cutting elements with ultrahard material because most brazing alloys that provide sufficient shear strength for bonding cutting elements to a drill bit also require brazing at temperatures above 1350° F. Therefore, alloys suitable for brazing cutting elements with ultrahard material thereon have been limited to only a couple of alloys which offer low enough brazing temperatures to avoid damage to the ultrahard layer and high enough braze strength to retain cutting elements on drill bits. These alloys generally require brazing temperatures close to or at 1350° F.
Brazing at high temperatures, such as around 1350° F. has also been found to result in a build up of residual stresses in the ultrahard material which can increase the chances of cutting elements cracking and chipping during drilling. Residual stresses occur primarily due to difference in the coefficient of thermal expansion and differences in thermal conductivity of the diamond and the catalytic material used to form a polycrystalline diamond compact (or other diamond containing component). In addition to the residual stresses, polycrystalline diamond has a tendency to gradually revert back to carbon (graphite), which is accelerated at higher temperatures. As a result, cutters brazed at higher temperatures typically have a higher chance of cracking and chipping in the field because of the higher stresses developed due to exposure to brazing temperatures. Therefore, a braze alloy that can be used to braze cutting elements at a reduced brazing temperature is desired.
Additionally, in some drill bit designs, the bond formed between the cutting elements and the cutting element support structure (e.g., blades, cones, bit body surface, etc.) may be the weak link in the bit design. This is a primary reason for cutting element loss, which can result in over loading of adjacent cutting elements which can lead to premature failure of the bit. Additionally, when a cutting element falls out of its support structure on the bit, the fallen cutting element may get jammed against the bit, causing further bit damage and accelerating the premature failure of the bit. Accordingly, there is also a desire for a braze alloy offering increased braze strength to reduce the risk of cutter loss in some drill bit designs and, in some cases, to allow for the development of more aggressive drill bit designs.